TOPIC: Sharing an ISP with 2 department

Hi ,
i would like to share 1 internet connection with 2 deferent department without both department seeing each other (something like vlan) but by using 2 standard routers. i have tried changing the 2 routers to different subnet mask 255.255.255.0 and 255.255.255.252 both wont be able to see each other in the my network places but it still can ping each other. is there any other way without using layer3 switches and highend routers.

well 1st router is connected to the modem to get a pppoe connection with the IP 172.16.0.1 SM 255.255.255.0 and the 2nd router is connected by pluging the wan interface of the 2nd router to the lan interface of the 1st router. Configured wan connection type as static and lan ip is 192.168.1.1 sm 255.255.255.252 .Both router can still ping each other. BTW for example if i setup a VLan using a L2 switch with this 2 rourter is it posible for the clients to view each other